1. Field of the Invention
The present invention relates generally to novel systems and methods for polymerizing nanoparticles within a polymer matrix and to novel methods for fabricating materials containing nanoparticles in a polymer matrix.
2. Description of the Related Art
Polymers are substances made of “many parts”. In most cases the parts are small molecules which react with each other to form a larger molecule having hundreds, thousands, or millions of the small molecules linked together. A molecule used in producing a polymer is a monomer. A polymer made entirely from molecules of one monomer is referred to as a homopolymer. Chains that contain two or more different repeating monomers are copolymers.
The resulting molecules may be long, straight chains, or they may be branched, with small chains extending out from the molecular backbone. The branches also may grow until they join with other branches to form a huge, three-dimensional matrix. Variants of these molecular shapes are among the most important factors in determining the properties of the polymers created.
The size of polymer molecules is important. This is usually expressed in terms of molecular weight. Since a polymeric material contains many chains with the same repeating units, but with different chain lengths, average molecular weight must be used. In general, higher molecular weights lead to higher strength. But as polymer chains get bigger, their solutions, or melts, become more viscous and difficult to process.
Life as we know it could not exist without polymers. Proteins, with large numbers of amino acids joined by amide linkages, perform a wide variety of vital roles in plants and animals. Carbohydrates, with chains made up of repeating units derived from simple sugars, are among the most plentiful compounds in plants and animals. Both of these natural polymers are important fibers. Proteins are the basis for wool, silk and other animal-derived filaments. Cellulose as a carbohydrate occurs as cotton, linen and other vegetable fibers. The naturally occurring form of the base polymers limits the properties of these fibers. Some, like linen and silk, are difficult to isolate from their sources, which makes them scarce and expensive. There are, of course, many other sources of proteins and cellulose. Wood pulp is an example of a cellulose source. Natural polymers, however, tend to be very difficult to work with and form into fibers or other useful structures. The inter-chain forces can be strong because of the large number of polar groups in the molecular chains. Thus, natural polymers usually have melting points that are so high that they degrade before they liquefy.
The most useful molecules for fibers are long chains with few branches and a very regular, extended structure. Thus, cellulose is a good fiber-former. It has few side chains or linkages between the sugar units forcing its chains into extended configurations. However, starches, which contain the same basic sugar units, do not form useful fibers because their chains are branched and coiled into almost spherical configurations. Synthetic polymers offer more possibilities, since they can be designed with molecular structures that impart properties for desired end uses. Many of these polymers are capable of dissolving or melting, allowing them to be extruded into the long, thin filaments needed to make most textile products.
Synthetic polymer fibers can be made with regular structures that allow the chains to pack together tightly, a characteristic that gives filaments good strength. Thus, filaments can be made from some synthetic polymers that are much lighter and stronger than steel. Bullet-proof vests are made from synthetic fibers.
There are two basic chemical processes for the creation of synthetic polymers from small molecules: (1) condensation, or step-growth polymerization and (2) addition, or chain-growth polymerization.
In step-growth polymerization, monomers with two reactive ends join to form dimers (two parts joined together), then “trimers” (three parts), and so on. However, since each of the newly formed oligomers (short chains containing only a few parts) also has two reactive ends, they can join together; so a dimer and a trimer would form a pentamer (five repeating parts). In this way the chains may quickly great length achieve large size. This form of step-growth polymerization is used for the manufacture of two of the most important classes of polymers used for textile fibers, polyamide (commonly known as nylon), and polyester.
There are many different commercial versions of polyester in a wide variety of applications, including plastics, coatings, films, paints, and countless other products. The polymer usually used for textile fibers is poly(ethylene terephthalate), or PET, which is formed by reacting ethylene glycol with either terephthalic acid or dimethyl terephthalate. Antimony oxide is usually added as a catalyst, and high vacuum is used to remove the water or methanol byproducts. High temperature (>250° C.) is necessary to provide the energy for the reaction, and to keep the resultant polymer in a molten state.
PET molecules are regular and straight, so their inter-chain forces are strong—but not strong enough to prevent melting. PET chains are long and “rigid” and their inter-chain forces, while somewhat strong, do not allow for significant alignment of groups on the chain that would interact strongly with other chains. In contrast, with cellulose the inter-chain forces are almost as strong as the hydrogen bonding that occurs in water. Thus, PET is a “thermoplastic” material; that is, it can be melted and then solidified to form specific products. Since its melting point is high, it does not soften or melt at temperatures normally encountered in laundering or drying. Another important property of PET is its Tg, or “glass transition temperature”. When a polymer is above its glass transition temperature, it is easy to change its shape. Below its Tg, the material is dimensionally stable and it resists changes in shape. This property is very important for textile applications because it allows some fibers, and the fabrics made from them, to be texturized or heat-set into a given shape. This can provide bulk to the yarn, or wrinkle resistance to the fabric. These set-in shapes remain permanent as long as the polymer is not heated above its Tg. Because its chains are closely packed and its ester groups do not form good hydrogen bonds, polyesters are also hydrophobic (i.e., they do not absorb water). This property also requires special dyeing techniques.
There are also many important classes of synthetic polyamides (nylons) and they have a wide variety of commercial uses. These are usually distinguished from each other by names based on the number of carbon atoms contained in their monomer units. As with polyesters, polyamides are formed by step-growth polymerization of monomers possessing two reactive groups. Here, the reactive functions are acids and amines. The monomers used may have their two reactive functions of the same chemical type (both acids, or both amines), or of different types. Thus, nylon 6,6—a very common fiber polymer—is made by reacting molecules of adipic acid (containing six carbons in a chain, with an acid function at each end) with hexamethylene diamine (also six carbon atoms, with amine functions at each end). In another variant the diamine contains ten carbons atoms, the product designated nylon 6,10.
The other common polyamide fiber polymer is nylon 6. Its monomer has six carbons in the chain, with an amine at one end and an acid at the other. Thus only one form of monomer is needed to carry out the reaction. Commercial production of nylon 6 makes use of caprolactam, a derivative that provides the same result.
As with the polyesters, nylons have regular structures that permit good inter-chain forces, imparting high strength. Both nylon 6 and nylon 6,6 have melting points similar to PET, but have a lower Tg. Also, since the amide functions in nylon chains are good at hydrogen bonding, nylons can be penetrated by water molecules. This allows them to be dyed from aqueous media, unlike their polyester counterparts.
In addition to nylon, there is another commercially important group of synthetic polyamides. These are the aramids, which contain aromatic rings as part of their polymer chain backbone. Due to the stability of their aromatic structures and their conjugated amide linkages, the aramids are characterized by exceptionally high strength and thermal stability. Their usefulness for common textile applications is limited by their high melting points and by their insolubility in common solvents. They are expensive to fabricate, and they carry an intrinsic color that ranges from light yellow to deep gold.
The polyurethanes, another group of step-growth polymers, are produced by the reaction of polyols and polyisocyanates. For fiber purposes, this class of linear polymers is formed from glycols and diisocyanates. Usually, the reactions are carried out to form block copolymers containing at least two different chemical structures—one rigid, and the other flexible. The flexible segments stretch, while the rigid sections act as molecular anchors to allow the material to recover its original shape when the stretching force is removed. Varying the properties of the segments, and the ratio of flexible to rigid segments controls the amount of stretch. Fibers made in this way are classified as spandex and they are used widely in apparel where stretch is desirable.
Chain-growth polymerization occurs when an activated site on a molecule, such as a free radical or ion, adds to a double bond, producing a new bond and a new activated location. That location then attacks another double bond, adding another unit to the chain, and a new reactive end. The process may be repeated thousands, or millions, of times, to produce very large molecules. This is usually a high energy process and the intermediate species are so reactive that, in addition to attacking available monomer, they also may attack other chains, producing highly branched structures. Since these branches prevent the molecules from forming regular structures with other molecules, their inter-chain forces are weak. The resulting polymers tend to be low-melting and waxy.
The breakthrough in making chain-growth polymers useful for fibers and for most commercial plastics came with the development of special selective catalysts that drive the production of long, straight polymer chains from monomers containing basic carbon-to-carbon double bonds. Ethylene and propylene form the simplest chain-growth polymers. Since their polymer chains contain no polar groups, these polyolefins must rely on close contact between the molecular chains for strength. Thus, the physical characteristics of polyethylene are very sensitive to even a small number of chain branches. Very straight chains of polyethylene can form strong crystalline structures that exhibit exceptional strength. Protective fabrics made from this type of highly structured polyethylene are virtually impossible to penetrate or cut.
Polypropylene is more complicated. Even without chain branching, each monomer unit adds one methyl group pendant to the chain. The arrangement of these side groups is described as the “tacticity” of the polymer. A random arrangement is considered “atactic”, or without tacticity. Regular arrangement with all side groups on one side of the chain is “isotactic”, and a regular alternating structure is “syndiotactic”. Polypropylene molecules can only pack closely in an isotactic arrangement. Synthesis of these polymers was a major challenge, but several stereoselective catalysts are now available, and high-density polypropylene has become a commodity product. Fibers made from it are lightweight, hydrophobic and highly crystalline. Their resistance to wetting gives them good moisture wicking and anti-staining properties. This also makes them virtually undyeable, except when the dye is applied to the polymer in its molten state—a process know as solution dyeing.
By contrast, the pendant nitrile functions in polyacrylonitrile are sufficiently polar to produce very strong inter-chain forces. Pure homopolymers from acrylonitrile are non-thermoplastic and difficult to dissolve or dye. Thus, for most commercial acrylonitrile polymers, small amounts of other monomers with bulky side chains are introduced to force the chains apart, to reduce the inter-chain forces. Common co-monomers for these fiber applications include vinyl chloride, vinyl acetate, acrylic acid, and methyl acrylate.
There are also a number of complex, specialty fiber polymers with methods of synthesis that are not easily classified. These materials are occasionally used in high performance materials where the complex structures impart exceptional strength, thermal stability, electrical conductivity, and others desirable properties. They include PBI (polybenzimidazole) and sulfur.
Most synthetic and cellulosic manufactured fibers are created by “extrusion”—forcing a thick, viscous liquid (about the consistency of cold honey) through the tiny holes of a device called a spinneret to form continuous filaments of semi-solid polymer.
In their initial state, the fiber-forming polymers are solids and therefore must be first converted into a fluid state for extrusion. This is usually achieved by melting, if the polymers are thermoplastic synthetics (i.e., they soften and melt when heated), or by dissolving them in a suitable solvent if they are non-thermoplastic cellulosics. If they cannot be dissolved or melted directly, they must be chemically treated to form soluble or thermoplastic derivatives. Recent technologies have been developed for some specialty fibers made of polymers that do not melt, dissolve, or form appropriate derivatives. For these materials, the small fluid molecules are mixed and reacted to form the otherwise intractable polymers during the extrusion process. Most manufactured fibers are produced using a spinneret, a metal cap with minute holes, through which a polymer is forced under pressure to produce fibers.
The spinnerets used in the production of most manufactured fibers are similar, in principle, to a bathroom shower head. A spinneret may have from one to several hundred holes. The tiny openings are very sensitive to impurities and corrosion. The liquid feeding them must be carefully filtered (not an easy task with very viscous materials) and, in some cases, the spinneret must be made from very expensive, corrosion-resistant metals. Maintenance is also critical, and spinnerets must be removed and cleaned on a regular basis to prevent clogging.
As the filaments emerge from the holes in the spinneret, the liquid polymer is converted first to a rubbery state and then solidified. This process of extrusion and solidification of endless filaments is called spinning, not to be confused with the textile operation of the same name, where short pieces of staple fiber are twisted into yarn. There are four methods of spinning filaments of manufactured fibers: wet, dry, melt, and gel spinning.
Wet spinning is the oldest process. It is used for fiber-forming substances that have been dissolved in a solvent. The spinnerets are submerged in a chemical bath and, as the filaments emerge, the filaments precipitate from solution and solidify. Because the solution is extruded directly into the precipitating liquid, this process for making fibers is called wet spinning. Acrylic, rayon, aramid, modacrylic and spandex can be produced by this process.
Dry spinning is also used for fiber-forming substances in solution. However, instead of precipitating the polymer by dilution or chemical reaction, solidification is achieved by evaporating the solvent in a stream of air or inert gas. The filaments do not come in contact with a precipitating liquid, eliminating the need for drying and easing solvent recovery. This process may be used for the production of acetate, triacetate, acrylic, modacrylic, PBI, spandex, and vinyon.
In melt spinning, the fiber-forming substance is melted for extrusion through the spinneret and then directly solidified by cooling. Nylon, olefin, polyester, saran and sulfur are produced in this manner. Melt spun fibers can be extruded from the spinneret in different cross-sectional shapes (round, trilobal, pentagonal, octagonal, and others). Trilobal-shaped fibers reflect more light and give an attractive sparkle to textiles. Pentagonal-shaped and hollow fibers, when used in carpet, show less soil and dirt. Octagonal-shaped fibers offer glitter-free effects. Hollow fibers trap air, creating insulation and provide loft characteristics equal to, or better than, down.
Gel spinning is a special process used to obtain high strength or other special fiber properties. The polymer is not in a true liquid state during extrusion. Not completely separated, as they would be in a true solution, the polymer chains are bound together at various points in liquid crystal form. Strong inter-chain forces cause liquid crystal alignments between molecules, resulting in an increased tensile strength of the fibers. In addition, the liquid crystals are aligned along the fiber axis by the shear forces during extrusion. The filaments emerge with an unusually high degree of orientation relative to each other, further enhancing their strength. The process can also be described as dry-wet spinning, since the filaments first pass through air and then are cooled further in a liquid bath. Some high-strength polyethylene and aramid fibers are produced by gel spinning.
While extruded fibers are solidifying, or in some cases even after they have hardened, the filaments may be drawn to impart strength. Drawing pulls the molecular chains together and orients them along the fiber axis, creating a considerably stronger yarn. It will be appreciated that filler materials, such as inorganic fillers, can be added to polymeric matrices, more specifically, to polymeric fibers so as to confer a variety of desired chemical and physical properties.
For example, U.S. Pat. No. 5,618,872, issued to Pohl et al. discloses the use of monodisperse, non-porous, spherical particles based on SiO2, TiO2, ZrO2, Al2O3, V2O5, Nb2O5 or mixed systems thereof, which are optionally modified on the surface by covalently bonded organic groups, as fillers in organic matrix materials, the refractive index of the particles being adapted to the refractive index of the organic matrix according to the use.
Nano-phase or nano-structured materials, i.e., nanoparticles or materials with grain sizes less than about 100 m (0.1 μm), are of great interest because such materials have properties different from and often superior to those of conventional bulk materials. Examples include greater strength, hardness, ductility, and sinterability; size dependent light absorption, greater reactivity among others. There has been considerable progress in determining the properties of nano-phase materials, small amounts of which have been synthesized (mainly as nano-size powders) by a number of processes including colloidal precipitation, mechanical grinding, and gas-phase nucleation and growth. Extensive reviews have documented recent developments in nano-phase materials. See Gleiter, H. (1989) “Nano-crystalline materials,” Prog. Mater. Sci. 33:223-315 and Siegel, R. W. (1993) “Synthesis and properties of nano-phase materials,” Mater. Sci. Eng. A168:189-197.
The potential applications of nano-phase materials include wear resistant coatings, thermal barrier coatings, ductile ceramics, new electronic and optical devices, and catalysts, among others. However, before the benefits of this emerging technology can be realized in the form of commercial products, two challenging problems need to be addressed, namely, (1) controlled, high-rate synthesis of nano-size powders, and (2) assembly of these powders into nano-structured materials. Controlled synthesis implies that the particles are uniform in size, composition and morphology, and are substantially unagglomerated, and generally requires that the consolidation or assembly be done in-situ to avoid contamination or to control the “nano-structure” of the resulting material. Nanoparticles have been made from metals (for example, Pd, Cu, Fe, Ag, Ni), intermetallics (for example, Al52 Ti48), and metal oxides (for example, TiO2, Y2O3, ZnO, MgO, Al2O3).
Interest in nanoparticles has grown over the last two decades because of the unusual properties these particles possess, properties that generally arise from the large surface area to volume ratios of the particles, but also from their size. Once formed, the nanoparticles can be used in a powder form, used as a coating material, or condensed into nano-phase materials. If denser nano-phase materials are desired than achievable through cold pressing, the nano-phase particles can be condensed using a hot pressing technique or sintered after the initial cold pressing step.
Nano-phase materials exhibit a variety of properties. For example, nano-phase metals have been reported with a yield stress and microhardness of three to five times greater than the same metals processed using conventional techniques. Nano-phase ceramics exhibit vastly improved ductility and malleability. One producer of nano-phase ceramics has demonstrated plastic, if not superplastic, deformability of a TiO2 nano-phase sample by pressing a cylinder of the material into a disk. The compressed disk did not exhibit any cracks or flaws.
Particle formation techniques include chemical and physical vapor deposition, mechanical attrition, gas phase pyrolysis and condensation, electrodeposition, cryochemical synthesis, laser pyrolysis, and gel synthesis. These techniques typically produce quantities on the order of grams per hour, quantities that are sufficient for research, but generally insufficient for most commercial applications.
However, such use of nanoparticles in polymeric matrices does not provide control of particle spacing within a polymer fiber. Accordingly, there remains a need for systems and methods by which nanoparticles can be controllably spaced within polymeric fibers.